A flash water heater using combustion of gas or oil has been available as a household item. This device features a quick rise in temperature and a large capacity; however, it causes air-pollution due to exhaust gas as well as combustion noises, and makes the users feel anxious about burning fuel at their places. On the other hand, a heat-pump water heater which stores hot water in a large tank for supplying the hot water, overcomes the problems due to the combustion, and features high heat efficiency because of using a heat pump. However, this water heater needs a large hot water storage tank, which imposes limitations on installation such as a weight and a space. There is an idea to overcome this problem, i.e., use of a heat pump in the flash water heater, however, the heat pump requires a time before heating up the water, this is a different point from a combustion water heater. Therefore, it takes time for a user to get hot water, and the user feels a complaint. In the case of the heat-pump water heater, the hot water supply changes depending on the natural condition such as temperature, humidity, and water temperature. And yet, a flowing amount of the hot water changes, so that it is difficult to restore quickly a hot water temperature to a certain level and cover a wide range amount of hot water supply. The heat-pump water heater thus has a problem in stable supply of hot water.
Japanese Patent Application Non-Examined Publication No. H02-223767 discloses a flash water heater using a heat pump for overcoming those problems. FIG. 11 shows a schematic diagram of this heat-pump water heater which includes heat-pump cycle 207 that couples compressor 202, radiator 203, decompressor 204, and heat absorber 205 via refrigerant flow path 201 to form a closed circuit. The heat-pump water heater further has the following elements:                heat exchanger 210 including water flow path 209 which exchanges the heat with refrigerant flow path 208 in radiator 203;        water supply pipe 211 for supplying water to water flow path 209;        hot water supply circuit 213 for coupling water flow path 209 to hot water supply terminal 212 such as a shower face or a spout;        temperature sensor 214 disposed at hot water supply circuit 213 for detecting a temperature of the hot water to be spouted; and        inverter 215 for controlling a rotation speed of compressor 202. Inverter 215 changes an output frequency to compressor 202 in response to a difference between a temperature detected by sensor 214 and a temperature set in advance. In other words, inverter 215 controls such that when a temperature of the hot water to be spouted is lower than the predetermined temperature, the rotation speed of compressor 202 is raised, the temperature of the hot water is higher than the predetermined temperature, the rotation speed of compressor 202 is lowered.        
This flash water heater undergoes unstable loads of hot water supply, in particular, a flow rate is varied by a user depending on a purposes of use, so that the load of hot water supply changes drastically. For instance, in the case of a home use water heater, the hot water supply for taking a shower or preparing a bath needs a flow rate as much as 10-20 L/min. However, the hot water supply for washing dishes or a face needs a flow rate as low as 3-5 L/min. A seasonal temperature change in tap water also one of the factors largely varying the load of hot water supply.
The load of hot water supply thus changes drastically depending on a change in flow rate or water temperature. On the other hand, a conventional heat-pump water heater, which controls the heat quantity of hot water supply by changing the rotation speed of the compressor in response to a difference between a temperature of the hot water to be spouted and a predetermined temperature, produces inconvenience of responsiveness to the control and stability of the control. For instance, if a control gain, a coefficient of a rotation speed of compressor and a temperature difference between the hot water and the predetermined temperature, is lowered for the more stable control, the compressor lowers its change in its rotation speed with respect to a change in the temperature difference. As a result, the temperature of the hot water changes only moderately and it takes time before the temperature reaches the predetermined one. The temperature of the hot water does not steadily rest at the predetermined one due to frequent changes by offset in flow rates or water temperatures. If the control gain is raised, a stable control can be expected for a large flow rate having a large load of hot water supply because a change in hot water temperature is small with respect to a change in the rotation speed of the compressor. However, in the case of a small flow rate, the temperature of hot water changes sharply in response to the change in the rotation speed of the compressor, so that the rotation speed control changes sharply, which does not stabilize the hot water temperature and also causes hunting due to phase-shift between a change in hot water temperature and a change in rotation speed. As a result, the control may be diverged.
The flash water heater employing a heat pump needs time for start-up of pressure and temperature in overall heat pump cycle before starting hot water supply, so that hot water supply from the water flow path in the heat exchanger delays comparing with that of a gas water heater. In a conventional structure, a difference between a hot water temperature and a predetermined temperature is only a factor to set a rotation speed of a compressor at starting the hot water supply. Therefore, the rotation speed of the compressor is set at a high level almost uniformly at the start-up when the hot water stays at a low temperature regardless of a large flow rate or a small flow rate. As a result, in the case of a small flow rate, the temperature of hot water supplied from the heat exchanger rises sharply, which causes an overshoot. This phenomenon produces inconveniences such that the hot water spouting becomes hotter than the predetermined temperature, and a rise of temperature of the radiator causes a pressure at the outlet of the compressor to rise abnormally.
Further, the conventional heat-pump water heater needs to change an operation of a single compressor 202 to alter the rotation speed; however, the control by changing the rotation speed of the single compressor has a limit to a controllable range. For instance, it cannot control such a wide range from a large flow rate for taking a shower and preparing a bath simultaneously in winter to a very small flow rate for washing dishes in summer. Thus some inconveniences occur such as lowering the temperature in taking a shower or spouting out too hot water in washing dishes.
Conditions of operating the heat pump cycle are changed due to the changes in temperatures of atmosphere and water and in the load of hot water supply, then the operating efficiency also changes. In the conventional heat-pump water heater, a rotation speed of the compressor is simply changed in response to a temperature of the hot water, so that an operating efficiency is not taken into consideration of the control. Thus the water heater is kept operating even in a low heating efficiency, and in some condition, the efficiency lowers so badly that the device cannot perform its capacity, and also the running cost becomes expensive.
On the other hand, in order to shorten a time needed for raise the temperature of hot water spouting at start-up, a new structure is proposed such that a hot water storage tank is disposed for exchanging the heat with the radiator, and the tank surrounds the compressor. This structure allows storing hot water in supplying hot water, and maintaining the compressor warm when the supply is stopped. Thus the temperature of hot water sharply rises when the supply restarts.
However, cooling off the tank lowers the temperature of the compressor, and the start-up of the heat pump cycle delays on the contrary because the tank deprives the compressor of heat. On top of that, since hot water is supplied from the tank, the hot water spouting becomes cool if the temperature of the tank lowers, and the hot water does not become hot until the temperature of hot water in the tank rises high. Therefore, it takes much more time before supplying hot water from a cooled storage tank.
The conventional heat-pump water heater, as discussed above, heats and controls the water regardless of the load amount of hot water supply, so that it is difficult to deal with a wide range of the load of hot water supply. It is also difficult for the conventional heater to be compatible with both of the responsiveness and the stableness to the control of hot water temperature, and yet, the heater sometimes delays the start-up of the hot water temperature, so that the efficiency of the heater is lowered.